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November 2008 | Volume 11/ Number 11
Ask the SIP Trunk Expert

SIP Trunk Solutions for Service Providers

There are two common ways that Internet Telephony (News - Alert)

Service Providers (ITSPs) connect customers to a SIP trunk: via a managed service where the service provider controls the connection from their Point of Presence to the enterprise edge, and directly over the public Internet. Both have enormous benefits; both require special attention to ensure security and interoperability.

Fortunately most of these issues can be resolved with the use of an enterprise Session Border Controller (E-SBC), which provides important security features and enables advanced SIP capabilities.

Common issues include the following:

Interfacing with all IP-PBXs: While service providers need to interface with as many IP-PBXs as possible, it is costly to achieve certification with every vendor. An E-SBC can be a normalization engine, connecting the PBX (News - Alert) to the service provider and supporting requirements for authentication and signaling.

Demarcation point: Many ITSPs want a clear hand-off point between their network and the end customer. The E-SBC serves this important function, delivering health and quality statistics while establishing a security boundary.

Security and NAT traversal: When connecting enterprises to SIP trunks directly via the Internet, carriers must resolve issues created by the enterprise firewall and traverse the NAT to connect to the customer’s Local Area Network (LAN) while also maintaining security. Again, an enterprise border element can provide the necessary functionality to resolve these problems.

Advanced Security: In addition to inspecting the SIP signaling and controlling the media ports, the E-SBC can add encryption to signaling and media (using TLS and SRTP), creating greater privacy. Some carriers offer this advanced level of security as a premium service aimed at the legal, financial or healthcare verticals.

Support for Remote Workers: For public Internet delivery implementations, the E-SBC can support remote workers who have access to the Internet. The E-SBC can resolve NAT traversal issues both at the enterprise edge and at the remote site. Resolution of the problem at the remote site requires only that far-end NAT traversal be enabled on the E-SBC and no additional hardware or software is required by the remote worker.

With a managed service, the ITSP hosts the SIP trunk separate from access to the public Internet. This separates the voice and data networks, and certain SIP features are not possible including support for remote workers. A border element can merge the two networks, maintaining QoS as well as opportunities for converged communications.

SIP trunking helps service providers to build their business today and generate future sales. With an edge device, the service provider can expand these opportunities and deliver a high quality, reliable SIP trunk to their customers. IT

Steven Johnson (News - Alert) is President of Ingate® Systems.

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